Tbhd, you said passing vehicles hear the whistle. Can you also hear it in the cab? I've never heard a whistle from ours nor has anyone ever mentioned a whistle. I have no idea what brand ours is. This is the second louvered tailgate we've had in almost ten years of use. This one we've had since 1999. No whistle that I've ever heard. I'm curious why yours whistles. I think I'll do a search and see if I can see what that brand looks like. Just curious.
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I found them at this url
http://www.handyindustries.com/tailgates.asp
There are two gates shown there. Which is yours? Both look like pretty standard FW tailgates. If yours was designed for your truck then I'm really curious why it whistles. We'll probably never know.